Gas turbine engine annular combustion chambers comprise an inner annular wall structure, an outer annular wall structure and an annular upstream end wall structure. The annular upstream end wall structure comprises an annular head and a plurality of heat shields. The heat shields are positioned downstream of the annular head and are secured to the annular head. The annular head is a cast structure and machined structure. The annular head is provided with a plurality of cooling apertures extending there-through to supply a coolant onto the upstream surfaces of the heat shields to provide impingement cooling of the heat shields. The heat shields are provided with pedestals on their upstream surfaces and/or have effusion cooling apertures extending there-through to provide further cooling of the heat shields.
The annular head also has a plurality of circumferentially spaced apertures, each one of which is arranged to receive a fuel injector. Each heat shield has a corresponding aperture arranged to receive a corresponding one of the fuel injectors.
The annular head, as mentioned previously, has a plurality of cooling apertures which have a diameter of 1.5 mm to 6.0 mm and these cooling apertures are conventionally machined through the annular head using electro-discharge machining (EDM) before the heat shields are fastened onto the annular head. These cooling apertures are drilled using electro-discharge machining (EDM) because the cooling apertures are drilled from the downstream side of the annular head and there is no easy access to the upstream side of the annular head to de-burr the cooling apertures. Electro-discharge machining (EDM) does not generate burrs and therefore if the cooling apertures are drilled using electro-discharge machining (EDM) no de-burring is required.
The electro-discharge machining (EDM) is a relatively slow method of drilling the cooling apertures when compared to conventional drilling, electro-discharge machining (EDM) is more prone to produce non-conformance of the cooling apertures when compared to conventional drilling and electro-discharge machining (EDM) is more prone to machine down time than conventional drilling.
However, conventional drilling produces burrs, or partially secured caps, at the exit of the cooling apertures on the upstream side of the combustion chamber head. These burrs, or partially secured caps, are difficult and costly to remove because there is no easy access to the upstream side of the annular head to de-burr the cooling apertures
Therefore the present disclosure seeks to provide a novel method and apparatus for producing cooling apertures in a combustion chamber head which reduces or overcomes the above mentioned problem.